Impedance Spectroscopy of VOCs Using PVD-Applied Sensor Substrates

1. Impedance Spectroscopy of VOCs Using PVD-Applied Sensor Substrates Martin Kocanda1,2, Michael Haji-Sheikh2, David S Ballantine1 Northern Illinois University, DeKalb, Illinois 60115 1Department of Chemistry and Biochemistry 2Department of Electrical Engineering Abstract — Conventional methods of fabricating anodic aluminum oxide (AAO) materials have utilized a multi-step anodization process to manufacture thick-film layers. In this investigation we have demonstrated that a single-step anodization process performed on PVD-applied metallic thin-films produces a nanoporous AAO material suitable for the adsorption, detection and discrimination of low molecular weight volatile organic compounds (VOCs). Electrical impedance spectroscopy methods have been employed to analyze the electrical response of the single-step anodized AAO materials in the presence of organic vapors. The sensor exhibits an impedance response that discriminates compounds based on the relative permittivity, polarizability and dipole moments of the analytes. Nanoporous AAO sensors exhibit a response that is modelled as a one-port reactive network containing a series resistive element (Rs) and an infinite number of passive RC devices (Rp and Cp) arranged in a series-parallel ladder topology. The response of the network is such that Rs is much smaller than Rp. Rs is usually ignored for low frequency measurements. Approximations have been made using a truncated series. Measurement techniques emphasize the response of the ladder elements as demonstrated in the results of this investigation and are consistent with the Cole-Cole dielectric model. Figure at left shows hexagonal Al grains before anodization. Center figures depict AAO domains and morphology at increasing magnification. Figure at right depicts pore geometry and spacing. Baseline temperature response Predicted response using Cole-Cole equation Future work includes the development of sensor arrays to sense and discriminate a suite of VOCs contained in a complex mixture and a fast-response sensor employed in the detection of various organic analytes in human breath samples emitted during normal respiration cycles. [1] Chen, Z., Lu, C., "Humidity sensors: a review of materials and mechanisms", Sensor Letters,. vol 3 pp 274-295, 2005. [2] Varghese, O.K., Gong, D., Paulose, M., Ong, K. G. Grimes, C. A. Dickey, E. C., "Highly ordered nanoporous alumina films: Effect of pore size and uniformity on sensing performance", Journal of Materials Research, vol 17(5), pp 1162-1171, 2002. [3] Haji-Sheikh, M., Andersen, M., Ervin, J., "Anodic nanoporous humidity sensing thin films for commercial and applications", proceedings, 39th IAS annual meeting, 2004. [4] Varghese, O.K., Gong, D., Dreschel, W. R., Ong, K. G. Grimes, C. A., "Ammonia detection using nanoporous alumina resistive and surface wave sensors", Sensors and Actuators B, vol. 94, pp. 27-35, 2003. [5] Radzik, C., Kocanda, M., Haji-Sheikh, M., Ballantine, D. S., "Electrical impedance response of a thick-film hybrid anodic nanoporous alumina sensor to methanol vapors", International Journal on Smart Sensing and Intelligent Systems, vol. 1(2), pp 470-479, 2008. [6] Kocanda, M., Haji-Sheikh, M., Ballantine, D. S., "Detection and discrimination of alcohol vapors using single-step anodized nanoporous alumina sensors", International Conference on Sensing Technology, Tianan, Taiwan, 2008. [7] CRC Handbook of Chemistry and Physics. 85th ed, ed. D.R. Lide, CRC Press LLC, 2005. Response to analytes at 20°C The sensor consists of interdigitated electrodes screen-printed over the nanoporous AAO layer. NIU CEET IAB 2010